best way to tell if floor is level/flat-and other flat surface issues!?

1)sorry if this is such a stupid question (I've searched the forum but searching for 'level floor' yields almost every post...What is the best way/reasonably affordable way to tell if the floor is level (or should I mean flat-whichever means the floor isn't uneven and is good for tiling...hmm isn't the word flat? Level means it has the same elevations? right?)(8 yr. old concrete slab) I have an aluminum yard level but this seems inadequate was going to go buy a longer one. I'm tiling a largish kitchen (about 15*20 or so)-recommendations?

2)also, for leveling/flatening the floor, I think I've noticed 2 small bumps in the floor(concrete slab) with just my little level...I'm renting a 9" grinder soon to get some thinset off part of the den floor that will be carpeted-can I just grind down the bumps on the concrete?
I know SLC is always a good choice but its $$$$-I've read thinset can be used for up to 1/4" variations-can it be used for more than this if you do it in 2 layers over time?

What about 'patching compounds'...are they more for pathing little divots in the floor (courtesy of the hilta electric demolition hammer)

3)What is the maximum allowable/recommended floor variation for ceramic tile?
If I used smaller tile (8-10 inch) would I be safer?

Get an 8' straight-edge from the big box stores. It will most likely come in (2) 4' sections that is sanpped together. ($14-$16)

Sure you can grind down high spots with a grinder.

If you choose SLC, you can shop around. There are SLC that go up to 1" and I think I saw one last week that will go to 1 1/2"

Little divots in the floor can easily be filled with thinset as you are tiling. They aren't much concern, unless they are big and deep.

Maximum allowable variations in the floor are 1/4" over 10'; and 1/16" in 1'. Meaning that the floor can vary up to 1/4" over a 10' straight-edge placed on the floor. And within 1', the floor should no vary more than 1/16".

Smaller tiles would hug the hills and valleys more if the variations were great. But if you fall within the above specs, you can tile with what you want and be done with it. If the floor varies a lot, SLC is a great way to make you life easier by allowing you to start with a flat floor.

You really need at least an 8' straightedge to do a good examination of your floor. And you are right, tile wants flat (basically straight in two dimensions) more than level (no slope).

While thinset can be used in multiple layers, you are better off using a good cement-based floor patch for larger areas. Ardex makes a good (and expensive) one. Home Despot carries one by Henry that consists of a bag of powder and a feather-edge latex additive. The additive is a bit pricey, but worth it, because the patch is stronger, and, as the name implies, it can be feathered out to nothing.

If you have a few obvious bumps, you should be able to use the grinder on them. Just don't try something like grinding the entire room flat.

I think that the magic number for tile is 1/4" in 10', but one of the pros will set me straight if I misspoke. Smaller tiles are easier to set on surfaces that are not flat because they will conform to the irregularities with less lippage.

Pro installers can flatten a floor by using more or less thinset under the tiles in different areas, but this is a difficult art to master. As a DIY you are much better off doing one thing at a time. First get the floor flat, and maybe level, too, and then set the tile on the already-flat surface. You'll wind up with a better floor, and will be much less likely to throw a bucket of thinset through the picture window.

wow thanks guys for all the info!
what is the ballpark range for slc per sq. ft. for reasonable coverage?
I have no idea how much SLC will be other than $$.

hmm while patching compound is expensive I think it might be the better way to go b/c I have about 300 s.q. ft. or so according to my rough rough calculations of tile to lay(entry way winding to a big kitchen and then a bathroom). Judging by the previous tilers work(and remembering correctly during my h@ll on earth jackhammering), the only place they did any real extra surface prep was by those 2 hump areas(elevated humps I believe..we'll see after the 8' tester tomorrow) so I figure that patching around the humps plus maybe some concrete grinding might smooth things out. what's the best way to apply patching compound around the humps? Should I just plan the application in the low spots using the 8' edge,
apply it manually with smaller tools and then use the same 8' edge and run it across to smooth and flatten out the top?

As much as I'm tempted to just pour massive amounts of SLC on the entire area I'm honestly scared to mess with the stuff, I figure it would probably be a lot more $$, and it doesn't seem like the previous tilers used the stuff much/any/at all

thanks again for all the tips..the problem is I think the floor is essentially level but has 2 elevated bumps (around 6"-12" areas) so if I filled in all the low spots woudln't that just mean filling in almost entire floor? Or should I just go for smoothing out some transitions?

Anywya I'm off to the house to finish picking out the mortar and tile fragments from the corners and such and then I'm gonna check to see exactly what sort of leveling issue I'm faced with and I'll report that back in

thanks guys!
I bought an aluminum 8' level (actually its labeled as a 'cutting guide') from lowes for around $20. I checked my floor and I think my floor is better than I thought. Not perfectly flat but I couldn't really find more than 1/4" variation in any spot on the floor. After lots of different placements I think I'm faced more with a 2-3' circular area of depression in front of the dishwasher and a 3' long *2' depression sloping down towards some cabinets with maybe a slight elevation on one side. None of these drop more 1/4" but they do change height greater than the 1/16"/1' rule so I'll fill them in with some thinset or maybe some pathing compound since I have a few littel dings (courtesy of the hilti jackhammer) that I need to tidy up. tHanks!!